Pressure-sensitive adhesive (PSA) tape has been widely used throughout the world for a variety of masking, holding, binding, protecting, sealing, marking, and other purposes. In one form, such a tape consists of a sheet-like carrier web having a coating of a normally tacky and pressure-sensitive adhesive layer. When the adhesive layer is releasable from the carrier web, the tape is often referred to as a transfer tape; such tapes are used to adhesively join two substrates. In addition to pressure-sensitive tapes, there are a variety of heat tackifying and heat hardenable tapes. Such tapes may be used, for example, in structural applications. Structural adhesive tapes include heat hardenable epoxy films that require refrigeration to extend shelf life.
Typically, an adhesive tape is designed for a broad, general use, or it is designed for a specific application. In either case, the selection of the adhesive is important. In a PSA tape, the adhesive is designed to provide bonding to a variety of substrates in a short length of time. This property of the adhesive is referred to as tack. An adhesive should also stay bonded and support a load over an extended time period. This property of the adhesive relates to its shear creep resistance. In adhesive tapes, it is also desired that the adhesive bond the tape backing to the substrate, or bond two substrates together, in the case of a transfer tape. The strength requirements of the adhesive bond varies with the application.
In general terms, pressure-sensitive adhesives consist of an elastic component modified through the addition of a viscous or plastic component. The initial bond is a function of pressure, hence the name "pressure-sensitive." To some extent, the initial bond or tack relates to wetting of the substrate by the adhesive. Good tack requires that the adhesive have considerable plastic deformation to relieve the stress in the adhesive caused by making it conform to the surface of the substrate. However, this same viscous or plastic behavior allows the adhesive to flow at intermediate stress levels over an extended time period (shear creep). Typically, adding more of the plastic component will increase tack and peel force, but will reduce shear creep resistance. Therefore, adhesive formulators are forced to make compromises in developing a PSA formulation for a particular application. See U.S. Pat. No. 4,260,659, Col. 1, lines 15-30, for a discussion of the difficulty in obtaining both good conformability and good shear properties in the same tape. In addition, variations in temperature and/or strain rate shift the viscoelastic behavior of the adhesive, and further complicate the ability to provide desired levels of tack, peel force, and shear creep resistance.
The peel force for PSA tapes often increases during the first day after the bond is made, but tends to level off after two days, with little further change thereafter. The rate of this increase in peel force is not the same for all adhesives. Increases in peel force may be related to the degree of wetting of the substrate by the adhesive. It may be expected that other factors being equal (such as surface energies) that adhesives having a high ratio of viscous to elastic deformation would wet a surface more easily. However, adhesives that have a large viscous component to their deformation (as compared to the elastic component) have less shear creep resistance, as well as less heat resistance. Heat resistance may be evaluated in terms of the temperature at which the stability of a bond is lost, and the bond fails.
Most heat tackifying tapes have little or no tack under ambient conditions, and therefore do not provide the desirable characteristics of a PSA, such as ease of use, and the ability to stay where placed (positioning convenience). Similarly, most heat hardenable or thermoset adhesive tapes and transfer tapes lack the positioning convenience of a PSA, and even when they do exhibit some room temperature tack, it is often necessary to heat the adhesives in these tapes to develop full strength. Prior to hardening or thermosetting, such adhesives may behave as a high viscosity fluid and flow, even under low stresses. Another consideration in formulating an adhesive tape is the variation in the resistance of the adhesive to solvents.
In an attempt to provide an adhesive tape with improved properties, some adhesive tape or tape-like products have been made with two adhesives. In one approach, layers of different adhesives have been coated on a carrier web. See, for example, U.S. Pat. No. 997,125 (Gleason), which shows a fabric coated with two continuous layers of heat tackifying adhesives, the outer layer having a lower melting point than the inner layer; U.S. Pat. No. 3,811,438 (Economou), and U.S. Pat. No. 3,885,559 (Economou) that teach coating two layers of PSA on a backing (one as stripes on an uninterrupted layer of the other) wherein the adhesiveness of the two adhesives differ; U.S. Pat. No. 4,112,177 (Salditt et al.) that teaches a porous backing with two or more layers of adhesive wherein the entire construction is porous; U.S. Pat. No. 4,260,659 (Gobran), U.S. Pat. No. 4,374,884 (Winslow), and U.S. Pat. No. 4,543,139 (Freedman et al.) that teach tape constructions that have two or more layers of adhesives. In addition, European Patent Application No. 0429269A1 (Calhoun et al.) discloses the use of PSA microspheres on the surface of an uninterrupted adhesive layer.
A second general approach to using more than one adhesive in a tape construction has been to coat a mixture of two adhesives. U.S. Pat. No. 3,326,741 (Olson) and U.S. Pat. No. 3,753,755 (Olson) teach mixing a PSA with a thermoset; and U.S. Pat. No. 4,049,483 (Loder et al.) teaches dispersing microspheres of PSA in a heat tackifying hot melt adhesive.
When one adhesive is coated on another where both are uniformly uninterrupted continuous layers, each adhesive is able to deform and flow as determined by the viscoelastic properties of each individual adhesive. Control of the composite properties of the tape is limited to variations in thickness of each adhesive layer, with no ability to continuously vary the area of coverage for each adhesive. When two or more immiscible adhesives are mixed, control of the composite adhesive properties of the adhesive tape is also limited to the specific manner of separation of the component adhesives.
Therefore, there is a need for a composite adhesive tape that provides better control over the physical properties provided by the respective adhesives. There is also a need for nontacky heat tackifying adhesive films that can be positioned by the use of a predetermined amount of PSA wherein the PSA is provided only on the surface of the otherwise nontacky heat tackifying adhesive film, and wherein the PSA is in a predetermined pattern. There is a need for a PSA tape or transfer tape that uses the viscoelastic properties of two or more adhesives, with the amounts and position of each adhesive predetermined, and each adhesive continuous through the thickness of the adhesive composite. There is also a need for an adhesive transfer tape wherein after transfer of the adhesive layer to an object to be bonded, the adhesive permits PSA positioning of the object on a vertical surface and the adhesive generates a structural bond during a subsequent thermal cycle without significant movement of the object.
There is a further need for a method for manufacturing composite adhesive tapes and transfer tapes that permits control over the amount and position of each adhesive.